Aircraft and industrial gas turbine engines include a combustor in which fuel is burned to input energy to the engine cycle. Typical combustors incorporate one or more fuel nozzles whose function is to introduce liquid or gaseous fuel into an air flow stream so that it can atomize and burn. General gas turbine engine combustion design criteria include optimizing the mixture and combustion of a fuel and air to produce high-energy combustion while minimizing emissions such as carbon monoxide, carbon dioxide, nitrous oxides, and unburned hydrocarbons, as well as minimizing combustion tones due, in part, to pressure oscillations during combustion.
However, general gas turbine engine combustion design criteria often produce conflicting and adverse results that must be resolved. For example, a known solution to produce higher-energy combustion is to incorporate an axially oriented vane, or swirler, in serial combination with a fuel injector to improve fuel-air mixing and atomization. However, such a serial combination may produce large combustion swirls or longer flames that may increase primary combustion zone residence time or create longer flames. Such combustion swirls may induce combustion instability, such as increased acoustic pressure dynamics or oscillations (i.e. combustion tones), increased lean blow-out (LBO) risk, or increased noise, or inducing circumferentially localized hot spots (i.e. circumferentially uneven temperature profile that may damage a downstream turbine section), or induce structural damage to a combustion section or overall gas turbine engine.
Additionally, larger combustion swirls or longer flames may increase the length of a combustor section. Increasing the length of the combustor generally increases the length of a gas turbine engine or removes design space for other components of a gas turbine engine. Such increases in gas turbine engine length are generally adverse to general gas turbine engine design criteria, such as by increasing weight and packaging of aircraft gas turbine engines and thereby reducing gas turbine engine fuel efficiency and performance.
Higher-energy combustion may also increase the temperature of the fuel nozzle assembly or combustor surfaces and structures, resulting in structural wear and performance degradation, such as fuel coking (i.e. build-up of oxidized fuel deposits) on fuel nozzle assembly surfaces. Fuel coking may lead to obstructions in fuel flow, such as within fuel injectors or along fuel-air mixing passages, which may reduce fuel nozzle efficiency or render the fuel nozzle inoperable. A known solution is to decrease fuel residence time within the fuel nozzle by reducing the area of a fuel circuit before injecting fuel into a fuel-air premix passage. However, such a solution obviates utilizing the fuel for secondary functions.
Therefore, a need exists for a fuel nozzle assembly that may produce high-energy combustion while further minimizing emissions, combustion instability, structural wear and performance degradation, and while maintaining or decreasing combustor size.